1. Field of the Invention
The present invention relates to a thermal conductivity substrate, a method of manufacturing the same, and a ceramic circuit board.
2. Description of the Related Art
Since an aluminum nitride sintered body (AlN sintered body) is excellent in heat dissipation properties, simplicity of a manufacturing method, influence to the environment, and electrical characteristics, the aluminum nitride sintered body is attracting attention as a substrate material for a ceramic circuit board. However, nitrogen in the AlN sintered body is eluted by the humidity or a chemical in the air, and the quality of the AlN sintered body is degraded. For this reason, the AlN sintered body has poor chemical stability.
Therefore, Jap. Pat. Appln. KOKAI Publication No. 63-170289 discloses a method of forming an oxide film on the surface of an AlN sintered body to chemically stabilize the AlN sintered body, i.e., a method of preventing elution of N (nitrogen) from the sintered body. However, this method requires strict control of the temperature and atmosphere to form a target oxide film. As a result, the manufacturing operation becomes cumbersome. In addition, cracks may occur in the oxide film due to the difference between thermal expansion coefficients of the oxide film and the AlN sintered body. Since the oxide film is bonded to the sintered body with a weak force, the oxide film is easily removed upon metallization on the oxide film.
Jap. Pat. Appln. KOKAI Publication No. 63-210005 discloses that the surface of a ceramic substrate such as an aluminum nitride substrate is toughened by hot phosphoric acid. The toughening process is performed using hot phosphoric acid which is obtained by heating strong phosphoric acid having a concentration of 75 to 90%, i.e., a pH of 1 or less, at 250.degree. to 360.degree. C. In addition, this publication discloses that the surface of the ceramic substrate is toughened by the hot phosphoric acid and metallized to manufacture a ceramic circuit board.
However, in this method, since the surface of the ceramic substrate is toughened by the hot phosphoric acid, the following problem is posed. For example, when a conductive pattern is to be formed on the surface of the ceramic substrate by a direct copper bonding (DCB) technique, after a copper plate is placed on the toughened surface of the ceramic substrate, they are heat-treated in a predetermined oxygen atmosphere at a high temperature at which a eutectic liquid phase between oxygen and copper is produced, so as to directly bond the copper plate to the ceramic substrate. In this process, a stress generated by the difference between the thermal expansion coefficients of the ceramic substrate and the copper plate acts on the interface therebetween. As a result, cracks are formed from the roughened part of the ceramic substrate. In addition, when a conductive pattern is to be formed on the surface of the ceramic substrate by a thick film technique, cracks are formed from the roughened part of the ceramic substrate when a conductive paste is sintered. When a conductive pattern is to be formed on the surface of the ceramic substrate by a thin film technique, a conductive thin film deposited by sputter deposition or the like cracks on the toughened surface of the substrate, and the conductive pattern is disconnected.